1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to a method and apparatus for initiating communications on a shared channel.
1. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a third-generation asynchronous mobile communication system using Wideband Code Division Multiple Access (WCDMA) based on General Packet Radio Services (GPRS) and a Global System for Mobile Communications (GSM) serving as a European mobile communication system.
In the Third-Generation Partnership Project (3GPP) responsible for UMTS standardization, Long Term Evolution (LTE) of the UMTS system is under discussion. The LTE is targeted for commercialization around 2010 and is a technology for implementing high-speed packet based communication at about 100 Mbps. For this, many methods are being considered. For example, there are methods for reducing the number of nodes on a communication path by simplifying a network structure, for enabling wireless protocols in close proximity to a radio channel, and the like. It is predicted that an LTE structure will be changed from a 4-node structure of the legacy UMTS system to a 2- or 3-node structure.
FIG. 1 illustrates an example of a structure of an evolved UMTS mobile communication system. As illustrated in FIG. 1, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified into 2-node structures of Evolved Node Bs (ENBs) 120, 122, 124, 126, and 128 and Evolved Gateway GPRS Serving Nodes (EGGSNs) 130 and 132. A User Equipment (UE) 101 connects to an Internet Protocol (IP) network 114 over the E-RANs 110 and 112.
The ENBs 120 to 128 are based on legacy Node Bs of the UMTS system and connect to the UE 101 through a radio channel. In comparison with the legacy Node Bs, the ENBs 120 to 128 perform more complex functions. Because all user traffics as well as a real-time service of Voice over IP (VoIP) are transmitted on a Shared Channel (SCH) in the LTE system, a device is required which can collect information of UEs and perform a scheduling process. The ENBs 120 to 128 are responsible for the scheduling process.
The term “evolved” is used to distinguish the 3GPP LTE system from the legacy UMTS system. To avoid the confusion in the following description, the terms “UE”, “Node B” and “network” are simply used.
The LTE system performs a Hybrid Automatic Retransmission Request (HARQ) between a Node B and a UE as in High Speed Downlink Packet Access (HSDPA) and Enhanced uplink Dedicated Channel (E-DCH). The HARQ is a scheme for increasing a probability of successful reception by soft combining previously received data with retransmitted data without discarding previously received data. However, because various Quality of Service (QoS) requirements cannot be satisfied only by the HARQ scheme, an outer ARQ can be performed in a higher layer. The outer ARQ is also performed between a Node B and a UE.
To implement a transmission rate of a maximum of 100 Mbps, the LTE system can employ a wireless access technology of Orthogonal Frequency Division Multiplexing (OFDM) at a bandwidth of 20 MHz. The UE can apply an Adaptive Modulation & Coding (AMC) scheme for setting a modulation scheme and a channel coding rate proper for a channel state.
In the LTE system constructed as described above, every data is transmitted and received on an SCH. A process for transmitting and receiving data on the SCH will be described with reference to FIG. 2. In FIG. 2, a receiver 205 and a transmitter 210 include a UE and a Node B in downlink, respectively, or a Node B and a UE in uplink, respectively. In the case of downlink communication as described below, the receiver 205 is the UE and the transmitter 210 is the Node B.
Before a packet is transmitted on the SCH in FIG. 2, the Node B 210 first transmits per packet control information on a Shared Control Channel (SCCH) in step 215. The per packet control information corresponds to a short Identifier (ID) of the UE 205 for receiving a packet, a packet size, a radio channel on which the packet is transmitted, a modulation scheme, channel coding, a HARQ, and the like. When receiving the per packet control information, the UE 205 determines whether its own short ID is equal to that included in the per packet control information and determines whether to receive a subsequent packet.
When the two short IDs are the same, the UE 205 receives a user data packet on the SCH, decodes the packet on the basis of the per packet control information, and performs an error check in step 220. The UE 205 transmits an Acknowledge (ACK) or Non-acknowledge (NACK) message to the Node B 210 on the basis of an error check result in step 225.
It is preferred that a size of the short ID of the UE is minimized because the short ID of the UE is information to be continuously transmitted on the SCCH (upon transmission of every packet). The short ID of the UE has a unique value within a cell. The Node B is responsible for allocating and de-allocating short IDs for UEs within a cell. Thus, when the UE is powered on or moves to a new cell, the UE desiring to transmit and receive data on the SCH should receive a new short ID allocated from a Node B of a current cell or the new cell.
A message for allocating the short ID cannot be transmitted on the SCH and therefore uses a new type of channel rather than the SCH in downlink. Next, an operation for receiving a newly allocated short ID will be described when the UE is powered on or moves to a new cell.
FIG. 3 is a message flow diagram illustrating a conventional operation for acquiring a short ID after a UE moves to a new cell.
Referring to FIG. 3, a UE 305 acquires system information from a Node B 310 of a current cell or a new cell when it is powered on or moves to the new cell in step 315. The system information is common information to be provided up to a cell boundary through known cell-by-cell channels, and includes information to be detected by the UE 305 for initiating communications in the cell. For example, the system information is random access information, neighbor cell information, and the like.
In step 320, the UE 305 transmits an Initial Uplink Message (IUM) on a Random Access Channel (RACH) using the acquired system information. In general, the IUM is used to notify a network of the presence of the UE 305, and contains a unique ID, capability information, and the like. When receiving the IUM, the Node B 310 sets a short ID to be allocated to the UE 305. In step 325, the short ID is contained in an Initial Downlink Message (IDM) and is transmitted to the UE 305. Before data is transmitted, the IDM is first transmitted from the Node B 310 to the UE 305. In addition, information required to use the SCH in the UE 305 is basically contained in the IDM. For example, the required information is about a Channel Quality Information (CQI) transmission scheme, transmission channel or HARQ configuration. The IDM is transmitted on a Forward Access Channel (FACH) rather than the SCH. The FACH serving as a downlink channel to be transmitted up to a cell boundary is not applied to the HARQ or AMC, which is different from the SCH. In step 330, the UE 305 receives the IDM, detects the short ID, and transmits and receives data on the SCH.
Accordingly, there is a need for an improved method and apparatus in which a UE initiates communications on a shared channel to initiate a data transmission and reception on the shared channel.